1. Field of the Invention
The present invention relates to a recording apparatus, and more particularly, it relates to a recording apparatus of the serial type, wherein at least a recording head is shifted by a stepping motor as a driving source to scan for a recording operation.
2. Related Background Art
In a conventional recording apparatus of serial type, a hybrid stepping motor or a PM (permanent magnet) stepping motor or a brushless motor has been used as a carriage driving motor for shifting a recording head to scan for a recording operation.
For example, in the brushless motor, generally, for example Hall elements have been used for detecting positions of magnetic poles of a rotor to perform an electric control, and an optical or magnetic encoder has been used for detecting the speed of the rotor.
However, such a conventional brushless motor has the following drawbacks:
(1) It is required that magnetic poles of a stator are correctly positioned with respect to the Hall elements; and
(2) Since the positions of the Hall elements and of the stator are determined unconditionally when the current change-over is effected by the Hall elements, a method for supplying current to the motor is limited to only one way. For example, since in case of a so-called 180.degree. electric control, the positions of the Hall elements regarding the magnetic poles of the stator differ by 45.degree. electrically from those in the case of a so-called 90.degree. electric control, if two kinds of electric controls are effected by a single motor, the number of the Hall elements will be increased twice and all of the Hall elements must be arranged in positions suitable for performing the respective electric controls.
Incidentally, the Japanese Patent Laid-Open Nos. 62-193548 and 62-193549 disclose a stepping motor wherein an electric control is effected by utilizing an encoder. However, these Patent Applications merely disclose the structure of the stepping motor itself including the encoder arranged in a predetermined position, but do not disclose or teach the control circuit or method for driving the stepping motor.
Now, the Applicant has proposed, in the U.S. Pat. Ser. No. 259,259 filed on Oct. 18, 1988, a control apparatus for a stepping motor wherein an encoder having detected portions the number of which is larger by a few times than that of the magnetic poles of the rotor is fixedly mounted on a shaft of the rotor, and when the rotor is rotated the number of the detected portions on the encoder passing through a predetermined position situated at the stator side is counted so that when the counted number coincides with a predetermined value the electric supply to the coil of the stator is initiated.
Conventionally, the drive control for the stepping motor has merely been performed by an open loop control treating the number of driving pulses of the stepping motor and the frequency of such pulse.
However, if the stepping motor is used as the carriage driving motor and the stepping motor is driven by the open loop control, during the movement of the carriage, discordant noise is generated due to the vibration of the rotor of the stepping motor, particularly, the hybrid stepping motor. Further, upon start, stop and reverse of the carriage, and accordingly, upon start, stop and reverse of the stepping motor, since the stepping motor is started o stopped with vibration, large noise is also generated. These noises must be avoided, particularly in an ink jet printer such as a bubble jet printer which generates no substantial noise.
Further, it can be considered that the above-mentioned brushless motor is used as the carriage driving motor. In this case, however, since the brushless motor has a long starting-up time upon start of the motor, it is not suitable for the carriage driving motor which requires the start, stop and reverse of the motor for each printing (recording) line, and therefore, the high speed printing or recording cannot be attained by the use of the brushless motor.
Now, in the U.S. Pat. Ser. No. 302,196 filed on Jan. 27, 1989, a recording apparatus has been proposed wherein the stepping motor is used as a driving source for shifting a recording head to scan for a recording operation and comprises a detecting means for an angular position of the rotor of the stepping motor, and a control means for closed-loop controlling the drive of the stepping motor in accordance with the detected result from the detecting means.
However, in order to closed-loop control the stepping motor, it is necessary to provide an encoder for detecting the angular position of the rotor of the stepping motor and it is also necessary to register the positions of the magnetic poles of the rotor with the positions of the magnetic poles (slits in the magnetic or optical system) of the encoder during assembling of the stepping motor. The reason why such registration of positions between the magnetic poles of the rotor and those of the encoder is required is that the phase change-over of the stepping motor must be synchronous with the output pulses of the encoder. If such positional registration is not obtained with high accuracy, the motor will not be rotated or will have different rotational speeds in opposite directions.
On the other hand, if the number of pulses generated during one revolution of the encoder is increased to improve the resolving power for each pulse, such positional registration will not be required. For example, in a PM stepping motor in which one revolution is achieved by 48 steps, the number of the magnetic poles of the rotor is 24 (twenty-four). In this case, if the number of the output pulses of the encoder is 288 for each revolution, the output having 12 (twelve) pulses can be obtained for each magnetic pole of the rotor. If the encoder is fixedly mounted on the shaft of the rotor at random, since the deviation between the center of the magnetic poles of the rotor and the center of the magnetic poles of the encoder corresponds to a half of a distance of two adjacent pulses at the most, such deviation will be included in the range of .+-.4.2%. In this case, the deviation in the change-over timing of the exciting current can be negligible.
However, in this case, it must be determined which magnetic pole of the encoder corresponds to the particular magnetic pole of the rotor. To this end, first of all, the current is supplied to the coils of the motor for a predetermined time or more. Then, when the rotor of the motor is slightly rotated by the energization of the coils due to such current supply and then is stopped, the magnetic pole in the encoder which is registered with the magnetic pole of the rotor is selected. The other magnetic poles in the encoder may be selected at intervals of twelve pulses on the basis of the firstly selected magnetic pole.
The initialization of the encoder as mentioned above must be effected prior to the action of the stepping motor. That is to say, when such stepping motor is used as the carriage driving motor for a serial printer, it is necessary to initialize the encoder before the printer is powered on.
However, since it is not ascertained where the carriage of the printer is positioned or stopped after the power source is turned OFF, the initializing operation can not often be performed correctly. For example, if the carriage is stopped at the right or left margin of its travel, or if the carriage is not further moved (and, thus, the motor can not be further rotated) in spite of the fact that some of the phases of the motor is energized for the initialization, or if the rotor is in a dead point (where an electric angle is deviated from the normal position by 180.degree. and the torque is zero), the initialization will be effected in a condition that the position of the rotor is not correctly set, with the result that the motor cannot be driven or may be overrun.
Further, when the stepping motor is driven or the rotor of the motor is held to perform the above-mentioned initializing operation, since the motor driving voltage is applied to the stepping motor as it is, it is feared that the excessive current flows through the stepping motor. To avoid this phenomenon, conventionally, a current control circuit as shown in FIG. 15 was prepared to limit the current in the motor drive operation and the rotor holding operation. Incidentally, in FIG. 15, the reference numeral designates a motor drive circuit.
However, when such current control circuit is incorporated, the construction of the motor drive circuit will be complicated, thus increasing the manufacturing cost, the number of parts, space of the substrate or the like.